This adds the entry point but does not implement it yet.

The entry point and enums are also added to the gl2ext_angle.h
header file.

BUG=angleproject:2775
TEST=angle_end2end_tests

Change-Id: I24c231c52e7cbb13637880b21044e655935b51e8
Reviewed-on: https://chromium-review.googlesource.com/1188575Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>